Hymenophyllaceae

Dubuisson, Jean-Yves, Pechon, Timothée Le, Bauret, Lucie, Rouhan, Germinal, Reeb, Catherine, Boucheron-Dubuisson, Elodie, Selosse, Marc-André, Chaussidon, Cécile, Dajoz, Isabelle, Pynee, Kersley, Grangaud, Edmond, Robert, Yannis, Tamon, Jean- Maurice & Hennequin, Sabine, 2018, Disentangling the diversity and taxonomy of Hymenophyllaceae (Hymenophyllales, Polypodiidae) in the Mascarene archipelago, with ecological implications, Phytotaxa 375 (1), pp. 1-58 : 44

Hymenophyllaceae and conservation biology in the Mascarenes

The montane forest (lacking on Mauritius) hosts a high diversity in Hymenophyllaceae with up to 15 taxa in the mesotherm and a maximum of 12 taxa for one locality. The ferns are also abundant in the understory covering most of the tree trunks as shown in ‘Avoune végétation’ and ‘Tamarinaie’ (see Fig. 3A, B ). This seems to be in accordance with the global observations of high diversity characterizing mesothermous montane and cloud forests at the pantropical level, especially concerning vascular epiphytes (as reviewed by Krömer et al. 2005; Kluge et al. 2006; Kluge & Kessler 2011). However, we here show that the maximum diversity for Hymenophyllaceae , at least in Mascarenes, is observed in the megathermous rainforest, as also shown locally for orchids ( Jacquemyn et al. 2005). This habitat hosts 21 of the 26 total taxa, and a maximum of 16 taxa for one locality, including 2 of the 3 endemic species and the 2 varieties restricted there. Megathermous ecosystems have been mostly replaced on La Réunion and Mauritius by crops and urban zones, and most remnants are invaded by exotic species ( Strasberg et al. 2005; Virah-Sawmy et al. 2009; pers. obs.). By replacing the indigeneous trees, which are potential phorophytes for the numerous epiphytic species, exotic species would likely have a negative impact on the growth of indigeneous epiphytes. Furthermore, by comparison with La Réunion and because the island is older, Mauritius could host at all of the 21 La Réunion taxa present in the megatherm zone. The six lacking taxa ( C. fallax , C. frappieri , H. sibthorpioides , H. capillare and its dwarf form, P. diaphanum , and V. gigantea ) present at the western Indian Ocean level would likely have been present in the original Mauritian rainforests and would have disappeared since the human colonization in the 17 th century. As examples, V. gigantea and H. sibthorpioides are reported in collections but have not been recently observed. Furthermore, many species have been reported recently or in collection from a single or two localities only: C. bonapartei , C. inopinatum var. tamonii , C. minutum var. mascarenense , C. trinerve , D. barklyanum , D. lorencei , H. fumarioides , and H. hygrometricum ; and this correspond to 53.3% of the current Mauritian species diversity. The Hymenophyllaceae , in addition to orchids, highlight therefore the importance of lowland ecosystems for the preservation of local biodiversity that should benefit from equivalent protections (if possible) as montane ecosystems. Most of the protected areas on La Réunion actually belong to the mesotherm and oligotherm zones within the National Park.

The leeward lowland semi-dry forests have also suffered from human activities and remain at only a few sites ( Strasberg et al. 2005). We show here that on La Réunion such threatened ecosystems host an unexpected diversity of hygrophilous plants, comprising at least 8 Hymenophyllaceae taxa (38% of the megathermous diversity). Semi-dry forests are crossed by numerous shady ravines with streams that maintain rocks permanently humid, even during the dry winter (see Fig. 4D ). Such shaded, wet rocks are suitable to host some Hymenophyllaceae populations that are usually also present in the rainforest at the same elevations. In addition, the present species are all small to dwarf (including the dwarfed C. inopinatum var. tamonii ) and as such appear to have lower hygrophilous requirements than large epiphytic species, growing as dense colonies on vertical or inclined rocks close to streams or waterfalls (see Fig. 9C and Fig. 11A ). Some emblematic species such as C. inopinatum var. tamonii and C. frappieri have most, or a significant part, of their populations located in such ravines in semi-dry forests with 68.2% and 38.5% of their populations, respectively. In addition, on the windward side, many megathermous species ( C. fallax , C. frappieri , C. inopinatum var. tamonii , D. barklyanum , D. cuspidatum , and D. lorencei ) are observed below 500 m as lithophytic on rocks in ravines, often in degraded forests close to urban zones and crops (see Appendices 1 & 3). Kluge & Kessler (2011) have already reported the high fern diversity in ravines compared to adjacent forests. The Hymenophyllaceae are not only emblematic of rainforests, but they also support the importance of the floristic diversity of wet ravines at the lowest elevations, that should mobilize the same conservation attention as other habitats.

Figures

FIGURE 3. Particular Mascarenan ecosystems. A. Hyperhumid montane ‘Avoune’ vegetation dominated by the endemic Erica reunionensis (Ericaceae), with most trunks and branches fully covered by epiphytes (liverworts, mosses, peat mosses, lycopods and ferns, especially here Hymenophyllum capillare and H. inaequale, and orchids) (La Réunion, ‘Pitons Mare à Boue’, ~1,600 m). B. Montane ‘Tamarinaie’ dominated by endemic Acacia heterophylla (Fabaceae), showing as for ‘Avoune’ tree-trunks and branches fully covered by epiphytes, including here H. inaequale (La Réunion, ‘Bélouve’, ~1,520 m). C. Remnant of leeward lowland semi-dry forest (La Réunion, ‘Ilet Solitude’, ~550 m). D. Typical wet ravines that host lithophytic Hymenophyllaceae on the wet and shaded rocks close to streams and waterfalls. (photographs.A: J.-Y. Dubuisson; B–C: J.-M. Tamon; D: A. Mercier).

FIGURE 4. Mauritian ecosystems. A. Lowland Sapotaceae rainforest, here invaded by the exotic Malagasy traveller’s trees Ravenala madagascariensis (Strelitziaceae) (‘Vallée d’Osterlog’ forest, ~260 m). B. ‘Pétrin’ locality showing particular habitat with thickets of the endemic Erica brachyphylla (Ericaceae) (~670 m) (photographs by J.-Y. Dubuisson).

FIGURE 9. A. Crepidomanes fallax (Christ) Ebihara & Dubuisson, as lithophyte on wet rock (La Réunion, ‘Rivière des Marsouins’). B. C. fallax, habit (from Madagascar, F. Rakotondrainibe 6467, P00248564). C. C. frappieri (Cordem.) J.P.Roux, covering a rock in a wet ravine (La Réunion, ‘Ravine Rivière Ste Anne’). D. C. frappieri, habit, type specimen (Bédier s.n., P00477825). E. C. frappieri, detail of long branched rhizome trichomes. F. C. frappieri, detail of sori (photographs.A, C & F: J.-M. Tamon).

FIGURE 11. Crepidomanes trinerve (Baker) Dubuisson & Ebihara. A. As lithophyte in a wet ravine of the western side of La Réunion (‘Les Canots’; photograph by J.-M. Tamon). B. Habit (from Comores, F. Rakotondrainibe 6651, P00310478; modified from Dubuisson et al. 2013).